The technology of cardiac stimulation devices has developed into a high level of sophistication with respect to system performance. The current generation of cardiac pulse generators, or pacemakers, incorporate microprocessors and related circuitry to sense and stimulate heart activity under a variety of physiological conditions. Cardiac pulse generators may also be of the type which include arrhythmia detection and may be programmed to control the heart in correcting or compensating for various heart abnormalities, e.g., antitachycardia pacemakers, cardioverters and defibrillators. A background description of modern cardiac pacemaker technology is set forth in U.S. Pat. No. 4,712,555, which patent is incorporated herein by reference.
As the complexity of cardiac pulse generators increases, the physician is faced with numerous programmable parameters to set and measurements to take at implant. It is because of this complexity that many physicians implant the pulse generators without ever reprogramming them. As a result, the manufacturer becomes the "implanting physician" by selecting "as shipped" values that are also considered safe for implanting. For example, a single-chamber VVI pulse generator is typically shipped by the manufacturer at a rate of 70 ppm, a ventricular output of 4 volts and 0.6 ms, and a ventricular sensitivity of 2 mV. If the pulse generator is shipped in the dual-chamber mode, the pulse generator will additionally include an atrial output of 4 volts and 0.6 ms, and an atrial sensitivity of 1 mV. Some manufacturers have elected to ship their dual-chamber DDD pacemakers in a VVI mode. If the physician never reprograms the latter devices to DDD mode or changes the output amplitude to conserve current drain, both the patient's health and the performance of the pulse generator are compromised.
Automatic features have been taught in several patents and incorporated into several manufacturer's devices. For example, "automatic output regulation" or "automatic capture detection" techniques typically include: automatically adjusting the energy of the applied pacing pulses according to a prearranged routine until capture is obtained; periodically testing the threshold (particularly during the acute phase); and verifying capture on a beat-by-beat basis, providing high amplitude backup pulses when capture is lost. See, for example, U.S. Pat. No. 4,729,376 (Decote, Jr.); U.S. Pat. No. 4,708,142 (Decote, Jr.); U.S. Pat. No. 4,686,988 (Sholder); and U.S. Pat. No. 4,969,467 (Callaghan et al.).
Autosensitivity features are also well known. For example, in U.S. Pat. No. 4,768,511 (Decote Jr.), the thresholds of the two voltage comparators are automatically adjusted so that one of the voltage comparators will sense the cardiac activity of the selected heart chamber and the other voltage comparator will not sense the cardiac activity. A similar arrangement is taught in U.S. Pat. No. 4,766,902 (Schroeppel). In U.S. Pat. No. 5,050,599 (Hoegnelid) two detectors are also used with a setting means which sets the sensitivity of the first detector such that the first detector means detects every event detected by the second detector, however, the second detector senses impedance which corresponds to an electrical cardiac signal while the first detector detects electrical cardiac signals. In U.S. Pat. No. 4,708,144 (Hamilton et al.), sensitivity is automatically controlled by measuring the peak value of each R-wave, and deriving a long-term average value. The gain of the sense channel is then adjusted according to the average of the measured peak values.
In U.S. Pat. No. 5,003,975 (Duncan et al.), an "Automatic Electrode Configuration Of An Implantable Pacemaker" is shown in which lead impedance is automatically measured to determine a functioning electrode configuration (unipolar, bipolar or unipolar from the ring). If a proper impedance measurement is not sensed for the programmed configuration, additional impedance measurements for other possible configurations are made in an ordered sequence in order to determine if an improper lead has been implanted or if an electrode has broken. When an operable configuration is found, the pacemaker continues operation in that configuration, thereby ensuring that capture can continue to occur until such time as the problem which has been detected can be corrected.
Another "automatic" feature is the "rate adaptive" or "rate-responsive" mode found in some pulse generators. Once this mode is enabled the pulse generator will automatically adjust the rate according to the patient's physiological needs, e.g., emotional and/or exercise demand.
A disadvantage of all of these "automatic" features is that they must be turned ON after implanting the device. They cannot be included in the "as shipped values" because in each case the device would be endlessly searching for a desired result or disable themselves. For example, a pulse generator in an autocapture mode would endlessly search for capture and/or perform a threshold search and/or disable itself when none was found. Likewise, the autosensitivity mode would endlessly look for cardiac signals that it could calibrate to. The automatic electrode configuration mode would never find an impedance to determine the "best" electrode to program to. The rate adaptive mode would be adjusting the rate according to the selected sensor (e.g., motion, temperature, etc.). Shelf-life is significantly affected by the dynamic current drain caused by the microprocessor constantly waking up to perform unnecessary pacemaker operations. Thus, it should be apparent that the current drain would be too excessive to permit shipping in these "automatic" modes.
What is needed is a pulse generator which will automatically turn itself ON at implant and automatically set itself to operate at safe thresholds for the patient, to initiate other "automatic" features," while still conserving power consumption, particularly during the "shelf-life" of the pulse generator. The present invention addresses these and other needs.